Vision lecture 2 Flashcards
(43 cards)
What are the properties of the LGN similar to and why?
Ganglion cell properties because they have centre-surround antagonism
How many layers does the LGN have?
6
How is the LGN organised? discuss
retinotopically - areas next to each other on your retina will be located next to each in your LGN
In the LGN, all cells are monocular, what does this mean?
- both eyes have inputs to LGN, but each eye goes to separate layers (the same layer each time)
- It either receives signals from the right eye or left eye but never both.
- layers 1, 4 & 6 get input from contralateral eye (opposite eye)
- layers 2, 3 & 5 from ipislateral eyen (eye on same side)
What are the two different types of layers in the LGN and how are they different?
Magnocelluar & Parvocellular & more recent koniocellular
- Magnocelluar: large, large receptive fields so can receive stimuli from a larger part of the retiner, can respond very quickly, higher sensitivity for detecting stimuli. It is better at everything, good at processing motion. Can’t detect colour
- Parvocelluar: small, smaller receptive fields, slower at responding & less sensitive, are well suited to processing colour particularly red/green
- koniocellular: very small, in between other two layers, don’t know what they do
How does the LGN carry neural signals to the primary visual cortex (v1)?
through optic radiations
Discuss the properties of the Primary Visual Cortex (V1)?
- retinotopic
- cortical magnification: more cortex devoted to fovea
- selectivity for orientation & eye-of-origin
What is selectivity for orientation in the Primary Visual Cortex (v1)
- all cells have a preferred orientation (what they will fire most to) in V1
- many “channels” selective to different angles
- reduced activity as orientation departs from the preferred
What is selectivity for eye-of-origin in the Primary Visual Cortex (v1)
- some cells are monocular (only receive stimulation from one eye) (1,7)
- others are binocular (2,3,4,5,6)
What do bincoluar neurons in the Primary Visual Cortex play a role in?
estimating depth
The range of orientations to which the cell fires lots is a measure of its what…. in regards to selecitivity of orientation in V1
bandwidth
small bandwidth > sharp tuning
large bandwidth > broad tuning
Discuss filters in regards to vision
- in vision, cells act as filters for many different stimulus properties
- they let the neural signal through for stimuli that they are tuned to, and filter it out for others
e. g. orientation filters, size (or spatial frequency) filters, colour (light wavelength filters)
Discuss in-depth how the primary Primary Visual Cortex is organised?
- V1 is organised into orientation columns
- in one column all the cells have same preferred orientation whether it be horizontal, vertical etc.
- also columns of ocular dominance - in some columns all cells take inputs from the same eye
- all of the above is repeated on every location on the retina
What are the three types of cells in V1?
- simple cells
- complex cells
- hypercomplex cells
What are simple cells (v1)
- respond to an oriented stimulus in a particular location within their receptive field
- can be bar detectors or edge detectors
What are complex cells (V1)
-respond to an oriented edge anywhere within their receptive field
What are hypercomplex cells (v1)?
- prefer stimuli with an end within the RF
- they care how long the stimulus is, will stop responding if you make the bar too long
- thus, if you encorach the bar into the surrounding field it will stop working
- known as “end-stopped”
What did Hubel & Wiesel show about building simple cells?
they showed that simple cells could, in principle, be “built” from the outputs of retinal ganglion cells (or LGN cells)
-you have to sum the inputs of gangion cells whose RFs are appropriately arranged
How do you build complex cells?
- need the output of a whole bunch of simple cells
- these simple cells need the same orientation preference
- but different edge locations
- we cannot these up using an OR gate so that the complex cell will fire if any of the simple cells will fire
How do you build hypercomplex cells?
- use the excitatory outputs from the complex cells and put inhibitory areas on the top and bottom
- you then connect it to a hypercomplex cell
- the output is reduced when you connect them to the top and bottom inhibitory zones
Discuss how you detect edges that aren’t actually there and where these cells are located
- the presence of an illusory edge could be signalled by interconnected hypercomplex cells
- located in V2
What does v1 care about in regards to its cells?
their orientation
What is the tilt aftereffect?
prolonged exposure to tilted bars makes subsequently seen vertical lines appear tilted in the opposite direction
What are the two explanations for the tilt after effect?
- through the two crucial properties of the visual system:
- orientation selective cells: (arranged in columns of cells with similar orientation preference)
- adaption: prolonged stimulation of same thing leads to a decrease in sensitivity and hence firing rate
